1. Field of the Invention
The present invention relates to an apparatus for and method of transmitting a video signal and, more particularly, to those suitable for dividing a video signal into a plurality of components by use of a Wavelet transform, etc. and coding them.
2. Description of the Related Art
Heretofore, in a so-called video signal transmission system for transmitting a video signal to a remote place as in the case of, e.g., a video conference system, for the purpose of utilizing a transmission line at a high efficiency, a transmission efficiency is enhanced by efficiently coding significant information with the aid of a correlation of the video signals.
Coding methods making use of this correlation have used orthogonal transform coding methods such as a predictive coding method, DCT (discrete cosine transform), etc. and a method of coding after dividing the video signal into a plurality of components such as a subband coding method and a Wavelet transform method disclosed in U.S. Pat. Nos. 5,014,134 and 5,068,911. The predictive coding method has a disadvantage by which a deterioration of picture quality is easy to be detected when a compression rate is increased, though the method is easy to implement as far as equipment is concerned and suitable for applying to coding of a relatively low compression rate.
Further, the orthogonal transform coding method such as DCT, etc. has been often used because of obtaining a high picture quality in a relatively easy manner at a high compression rate. However, the video signal is coded after being divided into blocks consisting of small area segments defined by, e.g., 8 pixels .times.8 lines. This causes the disadvantage that distortions conspicuously visible at boundaries between the blocks are produced, and an obstacle arising from a difference in degree of the distortion per block tends to be conspicuous to the eyes.
In contrast with this, a subband coding method and a Wavelet transform method have an advantage that the distortions at the block boundaries and fluctuations in distortion due to the blocks are less conspicuous than in a DCT method of performing coding per block because of dividing the video signal into components with respect to an entire picture.
For example, the Wavelet transform method among them is intended to code low frequency channel components per component obtained by recursive dividing in accordance with characteristics thereof. The processing with this Wavelet transform method applied two-dimensionally to the video signal is executed in a Wavelet transform circuit WT as shown in FIG. 1. The processing to reconstruct the original video signal, i.e., the picture from the divided components is executed in an inverse Wavelet transform circuit IWT as illustrated in FIG. 2.
This wavelet transform circuit WT is composed of a high-pass filter H for separating high-frequency components, a low-pass filter L for separating low-frequency components and a sub-sample unit (indicated by ".dwnarw." in the Figure) for effecting sub-sampling by multiplying a clock by 1/2. For instance, as illustrated in FIG. 3A, a low frequency channel part of a video signal VD for one-field consisting of 720 pixels .times.248 lines is recursively divided into components with three layers sequentially in vertical and horizontal directions. By this pieces of coefficient data A0, A1, A2, A3, B1, B2, B3, C1, C2, C3 as shown in FIG. 3B are obtained.
Contrastingly, an inverse Wavelet transform circuit IWT is composed of a high-pass filter H for reconstructing high-frequency components, a low-pass filter L for reconstructing low-frequency components, a sub-sampling unit (indicated by ".uparw." in the Figure) for effecting sub-sampling by multiplying the clock by 2 and an adder unit. For instance, the coefficient data AO, A1, A2, A3, B1, B2, B3, C1, C2, C3 divided as illustrated in FIG. 3B are reconstructed sequentially in the horizontal and vertical directions. The video signal VD for one field as shown in FIG. 3A is thereby obtained.
Such dividing and reconstructing of the components are attained typically by a finite length non-recursive type digital filter. In the Wavelet transform circuit WT, the low-pass filter L for separating the low-frequency components and the high-pass filter H for separating the high-frequency components are formed respectively of, as illustrated in FIGS. 4A and 4B, delay circuits D for one sample, a multiplier circuit for multiplying a signal delayed by each of the delay circuits D by a predetermined coefficient and an adder circuit for adding the multiplied results.
Further, similarly in the inverse Wavelet transform circuit IWT, the low-pass filter L for reconstructing the low-frequency components and the high-pass filter H for reconstructing the high-frequency components are constructed as illustrated in FIGS. 5A and 5B, respectively.
Video signal coding and decoding modules for coding and decoding the video signal by this Wavelet transform method are, as shown in FIG. 6, constructed. To be specific, in a video signal coding module 10, a video signal S1 inputted is divided into a plurality of components by a dividing circuit 11 consisting of the Wavelet transform circuit WT. The divided components are stored in a memory in the form of coefficient data S2 (A0, A1, A2, A3, B1, B2, B3, C1, C2, C3).
The coefficient data S2 (A0, A1, A2, A3, B1, B2, B3, C1, C2, C3) stored in the memory 12 are, as illustrated in FIGS. 7A and 7B, are blocked by combining the data for one pixel, 2 pixels .times.2 lines and 4 pixels .times.4 lines according to layers A, B and C. The blocked data are then read in a sequence shown in FIG. 8 and inputted as a coefficient data train S3 to a quantizing circuit (Q) 13.
The quantizing circuit 13 quantizes the coefficient data train S3 with a predetermined quantizing accuracy. Quantized data S4 obtained as a result of this quantization are inputted to a variable length coding circuit (VLC) 14. The variable length coding circuit 14 variable-length-codes the quantized data S4 inputted thereto by a method such as Huffman coding. Coded data S5 acquired as a consequence of this coding are multiplexed with quantization control signals S6 in a multiplexing circuit (MUX) 15.
Output data S7 of the multiplexing circuit 15 are transmitted via buffer memory (BM) 16 for smoothing the coded data quantity in the form of transmission data S8 composed of an output of the video signal coding module 10. At this moment, accumulated quantity data S9 representing an accumulated quantity of data in the buffer memory 16 are inputted to quantization control signal generating circuit (CONT) 17.
The quantization control signal generating circuit 17 creates a quantization control signal S6 indicating a quantization accuracy on the basis of the accumulated quantity S9 and a distribution of in-block electric power in the coefficient data train S3. The circuit 17 then supplies a quantizing circuit 13 with this signal. The quantization accuracy is thus controlled in accordance with the accumulated quantity of the buffer memory 16 and the distribution of the in-block electric power in the coefficient data train S3. The data quantity of the transmission data S8 can be thereby smoothed.
While in the video signal decoding module 20, to start with, transmission data inputted are stored in a buffer memory (BM) 21. An output of the buffer memory (BM) 21 is separated into coded data S12 and a quantization control signal S13 by a de-multiplexing circuit (DMUX) 22.
The coded data S12 thereof are variable-length-decoded into quantization data S14 by means of a variable length decoding circuit (VLD) 23 and inputted to an inverse quantization circuit (IQ) 24. The inverse quantization circuit 24 inverse-quantizes the quantization data S14 in accordance with the quantization control signal S13 inputted from the de-multiplexing circuit 22.
As a result, a coefficient data train S15 outputted from the inverse quantization circuit 24 is inputted to a rearrangement-oriented memory (MEM) 25, wherein the data train S15 turns out coefficient data S16 in an unblocked form shown in FIGS. 7A and 7B. The coefficient data S16 are inputted to a reconstruction circuit composed of an inverse Wavelet transform circuit IWT. The coefficient data S16 are reconstructed and decoded in this reconstruction circuit 26. A video signal S17 consisting of an output of the video signal decoding module 20 is thereby transmitted.
As described above, however, the picture quality often depends on the quantizing operation after being divided in the video signal coding and decoding modules 10 and 20 for coding and decoding the video signals S1 and S27 by the Wavelet transform method. In fact, roughly two kinds of methods are thinkable for the quantizing operation. Any method, however, presents the following problems.
That is, according to the first quantization operating method, a quantization accuracy is prescribed per component obtained by the Wavelet transform. The coefficient data S2 of the entire picture are uniformly quantized and variable-length-coded. With this processing, the distribution of the coefficient electric power differs depending on the video signal S1 inputted. Hence, there arises such a problem that the distribution of erroneous electric power inevitably fluctuates within the picture, with the result that a local deterioration of the image is easily detected.
On the other hand, according to the second quantization operating method, as shown in FIGS. 7A and 7B, the coefficient data S2 of the respective components obtained by the Wavelet transform is sub-blocked. The quantization accuracy is adjusted per sub-block, thus performing the quantization and variable length coding. With this processing, the distortion can be smoothed to some extent. It is, however, difficult to make uniform the distortion detected in the entire picture, for instance, if a fine pattern is provided in only an upper half of the picture, whereas a lower half is flat. This conduces to a problem, wherein the distortions are heavily distributed in the upper half.
Further, according to this method, the quantization coefficients of the respective components are variable-length-coded en bloc. Hence, an error correction having high correcting power is applied to the component exhibiting a high significance, whereas an error correction having low correcting power is applied to the components exhibiting a low significance. It is difficult to actualize an apparatus capable of transmitting a picture having the least picture quality through even ill-conditioned transmitting line by such applications. Additionally it is also difficult to actualize an apparatus, applied to fast-forwarding in, e.g., a video disc recorder, for extracting only the coded data of the components having the high significance from a bit train of the transmission data S8, S19 and simply decoding them.